Power distribution systems provide supply voltages to a variety of electronic systems, such as telecom and/or data communication equipment. In order to reduce start-up stresses and surge currents, power distributions systems may be soft-started to allow the output voltage to gradually increase until a steady-state operating point is achieved. In isolated power supply systems, synchronous rectifiers can be implemented in the secondary-side of a power transformer for efficiency.
There are two general solutions for pre-bias start-up of an isolated power supply system. One solution is to simply keep synchronous rectifier off until output voltage reaches to certain percentage of regulated voltage, usually about 90% to about 95% of the regulated voltage. Another approach utilizes closed-loop soft-start scheme and requires direct or indirect inductor current sensing. In this second solution, the synchronous rectifier is turned off right before inductor current goes negative to prevent undershoot of the output voltage and damaging MOSFET due to the negative inductor current. For an isolated system, especially with primary side control, it tends to be difficult and costly to obtain accurate inductor information. Additionally, the varying switching frequency introduced by pulse-skipping is not allowed in some applications.